Ceramic membranes containing certain multicomponent metallic oxide compositions demonstrate mixed conducting (i.e., oxygen ion conductivity and electronic conductivity) at elevated temperatures. These compositions used in these ceramic membranes, known in the art as mixed conducting multicomponent metallic oxides, may be employed in ion transport membranes and ion transport membrane systems for use in gas separation applications and partial oxidation applications.
Oxygen stoichiometry of these mixed conducting multicomponent metallic oxides is a thermodynamic function of temperature and oxygen partial pressure wherein the equilibrium oxygen stoichiometry decreases with increasing temperature and with decreasing oxygen partial pressure. The materials are chosen to provide high rates of oxygen transport and thermodynamic stability under process conditions.
The structural components of ion transport membrane systems are typically fabricated from metal alloys that may contain chromium, silicon, tungsten and/or molybdenum; and oxides of each of these elements. Contaminants including vaporized species such as CrO3, CrO2(OH)2, Si(OH)4, and WO2(OH)2 are known to evolve from hot metallurgy under oxidizing and steam environments at elevated temperatures. These contaminants may adversely react with or deposit onto apparatus used in systems employing ion transport membranes. Such species are believed to be generated by the following reactions:Cr2O3+2H2O (g)+ 3/2O2 (g)=2CrO2(OH)2 (g)  (a)SiO2+2H2O (g)=Si(OH)4 (g)  (b)WO3+H2O (g)=WO2(OH)2 (g)  (c)Cr2O3+ 3/2O2 (g)=2CrO3  (g).
Vaporized species such as chromium-containing species are known to evolve from the interconnects of solid oxide fuel cells (SOFCs) and react with the perovskite cathode material used in such SOFCs (Steel Research 72 (2001) 11, 528-533; Proceedings-Electrochemical Society (2001), 2001-16 (Solid Oxide Fuel Cells VII), 793-802; US 2003/0096147 A1) resulting in diminished cell performance over time. The SOFC field teaches that interconnects may be coated to prevent chromium from vaporizing and subsequently reacting with the perovskite cathode material used in such SOFCs. Such coatings must stay intact through repeated thermal cycles arising during operation of the SOFC to remain effective in preventing reaction between the vaporized species and the perovskite of the interconnects.
Ceramic membranes comprising a dense layer formed from mixed conducting multicomponent metallic oxides used in processes for producing synthesis gas (syngas) are known to experience flux decay and diminished flux performance when exposed to process streams containing elevated CrO2(OH)2, CrO3, Si(OH)4 and WO2(OH)2 partial pressures. Post-test analysis of these ceramic membranes revealed that the air-side surfaces of the membranes were coated with Cr-containing oxides while the syngas-side surfaces were coated with Si- or W-containing oxides. In some cases, the pores at the surface of the porous layer on the syngas-side of the ceramic membrane were almost completely plugged with such oxides.
Copending U.S. patent application Ser. No. 11/028,124, filed on Jan. 3, 2005, from which this application claims priority, teaches a method for gas purification comprising (a) obtaining a feed gas stream containing one or more contaminants selected from the group consisting of volatile metal oxy-hydroxides, volatile metal oxides, and volatile silicon hydroxide; (b) contacting the feed gas stream with a reactive solid material in a guard bed and reacting at least a portion of the contaminants with the reactive solid material to form a solid reaction product in the guard bed; and (c) withdrawing from the guard bed a purified gas stream. Suitable reactive solid material comprises one or more compounds selected from the group consisting of magnesium oxide, calcium oxide, copper oxide, calcium carbonate, sodium carbonate, strontium carbonate, zinc oxide, strontium oxide, and alkaline-earth-containing perovskites.
A need in the art exists for additional methods to minimize or substantially eliminate the presence of contaminants such as Si-, Cr-, Mo, and/or W-containing vapor species arising during operation of processes utilizing ceramic membranes formed from mixed conducting multicomponent metallic oxides and oxygen-conducting multicomponent metallic oxides. This need is addressed by the embodiments of the invention described below and defined by the claims that follow.